La2O3 reinforced Al based composite coatings by laser cladding

Optics and Laser Technology 117 (2019) 18–27

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Microstructures and properties of Ni/TiC/La2O3 reinforced Al based composite coatings by laser cladding

T

⁎

X. Hea,b,c, R.G. Songa,b,c, , D.J. Kongb,d a

School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou, Jiangsu 213164, China c Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China d School of Mechanical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China b

H I GH L IG H T S

have prepared Ni/TiC/La O reinforced Al based composite coatings on S355 offshore steel by laser cladding. • We the increase of scanning speed, the microstructure of the coating is improved obviously. • With coating has better mechanical properties and lower residual stress. • The • The protective effects of the composite coatings on the substrate corrosion resistance are significant. 2

3

A R T I C LE I N FO

A B S T R A C T

Keywords: Laser cladding Scanning speed Composite coating Microstructure Property

Ni/TiC/La2O3 reinforced Al based composite coatings have been prepared by laser cladding technique on offshore steel. Microstructure of coatings was investigated with scanning electron microscopy (SEM), a 3D microscope system, X-ray diffraction (XRD). The properties of composite coating have been tested and analysed by micro-hardness tester, friction and wear test machine and electrochemical workstation. The results showed that the coatings shows a good metallurgical bonding with the substrate. With the increase of the scanning speed, the microstructure of the coating was transformed from a short rod-like structure to granular structure. The pores and the coating dilution rate gradually decreased. The surface hardness of the coating reaches the maximum value of 847 HV0.2. The minimum wear width and wear depth was 449.4 and 15.5 μm, respectively, the wear rate was reduced to a minimum of 3.88 × 10−6 mm3 N−1 s−1. The coating has obvious passivation phenomenon, the pitting corrosion potential was higher, and the maximum impedance can reach 25 kΩ. When the scanning speed reached 7.5 mm/s, the performance of the coating was best.

1. Introduction The development and utilization of the ocean cannot be separated from the ocean steel, because the marine environment is extremely bad, the development of high-performance marine steel is the general trend of the times [1–3]. At present, the most effective way to improve the properties of marine steel is surface treatment, generally using surface coating technology [4]. The coating is required to achieve a long-term anticorrosion effect, the coating must be high-performance coating materials, as far as possible to reduce the number of coating maintenance, prolong the maintenance cycle [5]. Laser cladding is one of the most potential technologies to improve the corrosion resistance of metal surface in various surface coating processes [6,7]. With the

⁎

development of laser cladding technology, it is more and more widely used in marine engineering and other fields [8,9], and the cladding process does not produce pollutants, will not harm the marine environment [10]. Because Al coating has low electrode potential and good physical and chemical properties, so Al coating has certain anticorrosive properties. Laser cladding aluminum coating has become a mature protection technology for offshore and offshore steel facilities. The laser cladding aluminum coating shows excellent corrosion resistance to steel structures in splash zone at room temperature and high temperature after properly sealed cladding aluminum coating. Therefore, there are many researches on laser cladding Al-based coatings on steel at home and abroad [11–13]. However, the defects of low hardness, poor wear resistance and corrosion resistance are gradually

Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China. E-mail address: [email protected] (R.G. Song).

https://doi.org/10.1016/j.optlastec.2019.04.002 Received 17 January 2019; Received in revised form 20 February 2019; Accepted 6 April 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

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exposed, which is not conducive to the application in marine engineering. Therefore, how to improve the coating structure and enhance the coating performance has become a hot spot of current research [14]. Carbide ceramic powder not only has the characteristics of high melting point, high hardness and chemical stability, but also shows certain metal properties. It is an important material for the preparation of high wear resistance coating in the field of laser cladding [15,16]. It has been shown that rare earth powder La2O3 can refine the grain size, improve the microstructure of the coating, and enhance the high temperature oxidation resistance and corrosion resistance of the coating [17]. At present, Yang et al. prepared Al-TiC in-situ composite coating on the surface of AZ91D magnesium alloy by laser cladding. The results show that the coating has good metallurgical bonding with magnesium substrate, and the microhardness of the coating is 5–6 times higher than that of AZ91D substrate, and the wear resistance and corrosion resistance of the coating were significantly enhanced by adding mixed powders [18]. Sahoo et al. prepared Ni-TiC composite coating on AISI304 steel by laser cladding. The results showed that the microhardness and wear resistance of the coating were improved remarkably [19]. He et al. cladding laser-cladding Al-TiC-CeO2 composite coatings on S355 offshore steel, the coating and substrate achieve good metallurgical bonding, the microhardness of the coating is 3–4 times of that of the substrate, and the wear rate is about 3–4 times lower than that of the substrate [20]. Cui et al. studied the effect of scanning speed on the Ni-based composite coating. It was found that with the increase of scanning speed, the grain size of the coating was gradually refined, the crack tendency was increased, and the properties of the coating were gradually improved [21]. At present, it is common to use laser cladding technology to prepare high-performance Al-Ni-TiC composite coating, but Ni/TiC/La2O3 reinforced Al based coating is rare [22,23]. Therefore, in this paper, laser cladding technology was used to prepare composite coating with Al-Ni-TiC-La2O3 mixed powder as cladding material. The effect of scanning speed on the microstructure and properties of Ni/TiC/La2O3 reinforced Al based composite coating was studied by changing the scanning speed under the same condition. It provides some experimental basis for the application of Ni/ TiC/La2O3 reinforced Al based coating on offshore platform.

Table 2 Laser cladding process parameters.

The experimental powder materials were Al powder (purity 99.0%, average particle size 50–95 μm), Ni powder (purity 99.5%, average particle size 1.5 μm) and TiC powder (purity 99.5%, average particle size 40 nm). The above powders were mixed according to the mass ratio of 5:2:3 and then added with 1% La2O3 (purity 99.0%, average particle size 20 nm), The mixed powder was fully ground in a planetary ball mill for 12 h and then dried. S355 offshore steel was used as the base material and the contrast material in the test. The chemical composition of the S355 steel was shown in Table 1. Before the experiment, the substrate was cut into 60 mm × 30 mm × 5 mm rectangular lath by wire cutting machine. Grind the sample step by step with sandpaper, and finally cleaned it repeatedly with alcohol and acetone. Laser cladding experiment machine adopted ZKSX-2008 type 2 KW solid-state laser, cladding mode adopts feeding powder cladding, argon gas was used as power source of feeding powder and protective gas to realize synchronous feeding powder cladding. The process parameters of laser cladding are shown in Table 2. After completing the test, put the sample in a 500 °C incubator for 1 h. The morphologies, element compositions and phases of coatings

Mn

P

Cr

S

Ni

Zr

Fe

0.17

0.55

0.94

0.035

0.065

0.035

0.065

0.15

97.99

Powder feeding rate (g/min)

Argon gas velocity (L/ min)

Spot diameter (mm)

1200 1200 1200 1200

6 7 7.5 8

8 8 8 8

15 15 15 15

3 3 3 3

3. Results and discussion 3.1. Microstructures analysis of coatings Fig. 1 shows the morphology and EDS results of the mixed powder. It can be seen that the particle size of Al powder is large and irregular. In order to avoid agglomeration because the particle size is too small, so the particle size range of Al powder is from 50 μm to 95 μm [24]. Ni powder adsorbed on the surface of Al powder, the main shape is spherical and the particle size is about 1.5 μm. The particle size of TiC powder is small and spherical. La2O3 powder adsorbed on the surface of Ni powder, this is mainly due to van der Waals force and chemical bond interaction between particles [25], and the shape is irregular, the

Table 1 Chemical composition of S355 steel. Si

Scan speed (mm/s)

were analyzed using a JSM-6360LA type scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and D/max2500 PC X-ray diffraction (XRD), respectively. The microhardness of the cladding layer was measured by HMV-1T digital microhardness tester. The loading load is 200 g and the loading time is 15 s. A point was hit every 100 μm from the surface of the cladding layer to the substrate, and the average value was measured three times at the same depth level. The indentation of the coating was observed by optical microscope. The elements and compositions were analyzed by the EDS spectrometer attached to the scanning electron microscope (SEM). The residual stress was measured by the X350-A stress tester. The residual stress was measured by the roll-fixed method, the cross-correlation method was used to determine the peak value, and the target material was used as the Co target. The radiation source is Co-Kα1, the Bragg diffraction crystal plane is (400), the incident angle is 0°, 25°, 35°, 45°, respectively, the stress constant is −130 MPa/°, the starting and terminating angle range of 2θ scan is 155–145°, the scanning step of 2θ is 0.10°. Count time: 0.50 s, X-ray tube high voltage: 22.0 kV, X light tube current: 6.0 mA. The wear resistance of the coating was tested by CFT-1 comprehensive tester of material surface properties. The grinding material used was 45# steel at a loading of 200 g and a motor speed of 500 r/min, using a reciprocating sliding mode, the wear scar radius was 3 mm and the running time was 30 min. The wear medium was air and the measurement was completed using a BT25S electronic analytical balance to measure the weight loss. After completing the test, the wear samples were observed and analyzed by a VHX-6000 type 3-D microscope system. Electrochemical tests were carried out by using CS350 type electrochemical workstation. The sample dimension of electrochemical test was 10 mm × 10 mm × 3 mm. Except for the surface of the coating, the rests of the surfaces were inlaid with epoxy resin. The test medium was 3.5%NaCl solution. The electrochemical workstation system consists of saturated calomel electrode (SCE) as the reference electrode, Pt as the counter electrode, and the sample as the working electrode. The scanning rate of potentiodynamic potential was 1 mV/s, the sampling frequency was 0.5 Hz, the measuring potential range was −0.5 to 0.5 V, and the testing time was 1800 s. The frequency range measured by EIS was 10−1 × 105, and the test time was 300 s. All measurements were made after immersion in 3.5%NaCl solution for 30 min until the open circuit potential (OCP) was stable. The surface corrosion morphology of the sample was observed after the test was completed.

2. Experimental

C

Laser power (W)

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(a)

La2O3

Ni

(b) TiC

Al

Fig. 1. Morphologies (a) and EDS analysis (b) of powders.

time of alloy powder is, so the interaction time with substrate is reduced, the melting rate of substrate is decreased, and the thickness of coating is reduced [26]. Causes this dilution rate to decrease. Because the dilution rate directly affects the performance of the coating, the dilution rate is expressed by the Formula:

particle size is nano-level. Fig. 1b shows the analysis result of energy spectrum. It can be seen that the mass ratio of Al, Ni and TiC in the mixed powder is approximately 5:2:3, which is also consistent with the result of the total mass ratio of the added powder. Due to the limitation of the conditions, the prepared powder contains a certain amount of O element. Fig. 2 shows the cross-sectional morphology of the coating at different scanning speeds. The cross-section of the coating is divided into three regions in turn: the cladding layer, the heat affected zone (HAZ), and the substrate. The coating and the substrate show a good metallurgical bonding. When the scanning speed is low, the coating appears microcracks, and both the coating thickness and the dilution rate are large. When the scanning speed is further increased, the coating thickness and dilution rate are gradually reduced, and there is no obvious visible crack. The change in extent largely results from the fact that the spot diameter and laser power are fixed, the larger the scanning speed is, the smaller the specific energy Es is, the longer the melting

λ=

h × 100% H+h

(1)

where H is the thickness of the cladding layer and h is the melting depth of the substrate [27]. The dilution rates of the coatings prepared at four different scanning rates are 10.23%, 9.69%, 8.33% and 7.41%, respectively. It is generally believed that the dilution rate of the coating is about 6%, so when the scanning speed is 7. 5 mm/s, the dilution rate of the coating is the best. Fig. 3 shows the XRD spectra of the coatings prepared at different scanning speeds. When fixing the laser power and spot diameter, it can be seen from the figure that the cladding layer phase is basically the

Fig. 2. Morphology of coatings cross-section with different scanning speeds (a) 6 mm/s, (b) 7 mm/s, (c) 7.5 mm/s, (d) 8 mm/s. 20

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specific energy Es is larger and the interaction time between laser and powder is longer, which leads to the existence of the molten pool is too long, but the cooling rate of the molten pool is relatively small in the cooling process. The precipitated TiC has more time to grow up and form short rod-like structure. On the other hand, when the scanning speed is low, the diluent rate of the coating is higher, the residual stress is higher, the shrinkage of the coating is larger than that of the substrate when cooled to room temperature, the substrate is compressed, and the cladding layer is pulled, the formation of the cracks is based on such a result. With the increase of the scanning speed, the specific energy Es decreases gradually, the time of laser action on the coating surface also decreases, the cooling rate of the molten pool increases gradually, resulting in the precipitation of TiC cannot grow up and become the original state [30]. 3.2. Properties analysis of coatings The microhardness distribution from the cladding surface to the substrate as shown in Fig. 6a. It can be seen that the microhardness of the coating is much higher than that of the substrate. However, the hardness of the coating shows downtrend in the perpendicular to the laser scanning direction. Because in the solidification process of the molten pool, the solidification phase transition first occurs at the bottom, and then the solidification phase transition advances to the top, which makes the growth time of the bottom structure of the molten pool longer and the growth time of the top shorter, resulting in the relatively coarse structure of the bottom TiC, The microstructure of the top TiC is finer, which shows a decreasing trend of microhardness [31,32]. The average microhardness of the substrate surface is 325 HV0.2, The microhardness of the coatings surface prepared at four different scanning speeds was 728 HV0.2, 754 HV0.2, 847 HV0.2, 804 HV0.2, respectively, as shown in Fig. 6b. The coating can obviously improve the hardness of the substrate. With the increase of the laser scanning speed, the solidification rate in the molten pool is gradually accelerated. And the surface tension and critical nucleation radius of liquid metal are reduced by adding proper amount of rare earth oxides, and the supercooling of composition during solidification is avoided, and the segregation of composition is reduced, thus the microstructure is homogenized. The microhardness of the coating increases with the increase of the fine grain strengthening effect. However, the microhardness of the coating decreases slightly at 8 mm/s, which is mainly due to the existence of pores and cracks on the surface of the coating, which weakens the improvement of the microhardness to a certain extent. It can be seen from Fig. 7 that the residual stress in the cladding layer is tensile stress. With the increase of scanning speed, the residual stress increases and reaches the maximum at 7 mm/s. With the increase of scanning speed, the laser beam makes the temperature of the surface rise sharply in a very short period of time, and the heat cannot be transmitted around the material in time, which leads to the narrowing of the high temperature region, the increase of the inhomogeneity of the temperature, and the increase of the compression stress during expansion. Compression plastic deformation increases. When the scanning speed increases to a certain extent, the absorption energy of the molten pool decreases, the temperature decreases, the yield strength increases, and the plastic deformation is difficult to occur, residual stress decreases gradually [33]. Fig. 8 shows a curve of friction coefficient of substrate and coating with time in air. The wear process of the coating and substrate can be divided into two stages, the running in period and the stable wear stage. The friction coefficient of the substrate increases rapidly to about 1.2 in 0–3 min, then decreases to about 1.2, and finally the friction coefficient is stable at about 1.2, because a layer of oxide film may be formed on the surface of the substrate. Before the oxide film is scratched, the friction force increases gradually, so the friction coefficient increases rapidly, until 45 # steel breaks the oxide film and contacts with S355

Fig. 3. XRD diffraction pattern of coating by laser cladding.

same, mainly for the reinforcing phase TiC and some continuous phase (AlFe3 and AlNi3). AlFe3 phase was detected at about 35° and 82°, which indicated that Al and Fe not only diffused each other, but also combined to form a new phase in the coating, which improved the bonding strength of the interface of the coating and increased the reliability of the coating [28]. In addition, low crystalline diffraction peaks of Al2O3 phase were observed at 64° and 78°, indicating that there may be a slight oxidation in the coatings. At scan rates of 7 and 7.5 mm/s, the LaAl3 phase was detected in the coating. The possible reason is that rare earth oxide La2O3 with strong chemical activity, will undergo a variety of metallurgical reactions such as decomposition and combination in the molten pool under the radiation of high energy laser, thus forming a relatively stable rare earth compound LaAl3. Fig. 4 shows the three-dimensional topography of the coating surface and the scanning area of the graph in the lower right corner. When the scanning speed is 6 mm/s, the height of the wave crest up to 80.01 μm, and the surface roughness is very high. When the scanning speed are 7 and 8 mm/s, the wave peak height of the coating is close, but the roughness of the coating prepared by 7 mm/s is lower. When the scanning speed reaches 7.5 mm/s, the wave peak height decreases to 51.89 μm. The formation of the wave crest is mainly related to the fluidity of the molten pool. These wave peaks not only reduce the effective thickness of the coating, but also lead to the stress concentration in the wave zone, leading to the formation of cracks [29]. In a word, the lower the scanning speed, the higher the crack sensitivity and roughness of the coating. Fig. 5 shows the surface morphology of the coatings prepared at different scanning speeds. Fig. 5a shows the coating prepared at a scanning speed of 6 mm/s. There are many pores and a very long crack is distributed on the surface of the coating. The reinforced phase TiC shows coarse granular and short rod-like, which is the typical morphology feature of TiC. And TiC has the tendency of connecting and growing up. When the scanning speed is increased to 7 mm/s, the pores in the coating decrease and the cracks disappear, and the reinforced phase TiC is mainly short rod-like and grows regularly in a certain direction. When the scanning speed is 7.5 mm/s, the porosity in the cladding layer decreases further, and the short rod-like TiC disappears and becomes granular, but there is also a slight tendency of connecting growth. Because rare earth compounds can easily react with other elements to form stable compounds, the nucleation rate and quantity in the solidification process of the cladding layer are accelerated, and the grains are refined. When the scanning speed is further increased to 8 mm/s, the pores on the surface of the coating increase and smaller cracks appear. The TiC is mainly granular and dispersed, but the growth direction is not typical. When the scanning speed is low, the laser 21

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Fig. 4. The surface geometry of the coatings with different scanning speeds: (a) 6 mm/s, (b) 7 mm/s, (c) 7.5 mm/s, (d) 8 mm/s.

released by plastic deformation in the cladding layer. There is no brittle fracture of the body, so that the wear resistance is significantly improved and after adding La2O3, the LaAl3 phase is formed, which can strengthen the second phase, reduce the friction coefficient of the cladding layer and improve the wear resistance of the coating. Fig. 10 shows the polarization curves of coating and substrate in 3.5%NaCl solution at different scanning speeds. When the scanning speed is 6.0 mm/s and 7.0 mm/s, the self-corrosion potential of the coating close to −0.76 V, and when the potential range from −0.5 V to −0.1 V, the curve shows obvious passivation phenomenon. Due to the fact that the corrosion material deposited on the substrate surface hinders the dissolution of the metal. When the potential reaches −0.1 V, the passive film on the surface of the coating is punctured, and the corrosion current increases rapidly. Since the corrosion material deposited on the substrate surface hinders the dissolution of the metal [37]. When the scanning speed is 7.5 mm/s, the corrosion potential close to −0.6 V. When the polarization potential more than −0.5 V, the current density increases rapidly. When the scanning speed increases further, the corrosion potential decreases slightly. Table 4 is the result of electrochemical fitting of polarization curve in Fig. 10 by CorrView software. From the point of view of corrosion kinetics, higher corrosion potential and lower corrosion current indicate better corrosion resistance. It can be seen that when the scanning speed is 7.5 mm/s, the self corrosion potential is higher and the corrosion current density is lower. The corrosion speed (Vcorr) is defined as

steel directly. Because the hardness of 45 # steel is about HRC55, it is higher than that of S355 steel substrate. Compared with the matrix, the abrasive material is hard abrasive and can be easily inserted into the matrix to form micro-cutting, so the friction coefficient will gradually decrease and finally stabilize in a certain range at this stage [34–36]. The coating was in the wear stage within 5 min, and gradually returned to the stable stage after 5 min. Because the surface of the coating is flat and smooth, and its hardness is obviously higher than that of the opposite grinding material, the wear mode of the coating is mainly a small amount of scratching, so the overall fluctuation of the friction coefficient curve is relatively small. The average friction coefficients of the coatings are 0.834, 0.729, 0.567 and 0.622 respectively when the scanning speed is 6.0–8.0 mm/s. It can be seen that with the increase of the scanning speed, the friction coefficient of the coating has a decreasing trend, which is the same as the changing trend of microhardness. Fig. 9 shows the wear mark morphology of the coating. Based on the wear data of Table 3. When the scanning speed is 7.5 mm/s, the wear rate of the coating is the lowest and the wear resistance property is the best. The microstructure of the cladding layer plays a dominant role. Since the coating is mainly composed of intermetallic compounds, Al2O3 ceramic phase particles are dispersed in the microstructure of the composite coating. In the process of friction and wear, it can effectively hinder the plastic deformation of the substrate and play a certain supporting and bearing role, so can change the contact characteristics between the friction pair and the coating, thus reducing ploughing and adhesion, and improving the wear resistance of the coating. In addition, in the friction process, the force acting on the reinforced phase TiC is distributed on the ceramic phase with good toughness and the superelastic AlNi intermetallic compound by transfer, and then the stress is

Vcorr =

Micorr ηF

(2)

where icorr is the current density; M is the atomic weight of the metal; n is the valence of the metal; F is the Faraday constant. From the point of 22

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Fig. 5. Surface morphology of coatings with different scanning speeds: (a) 6 mm/s, (b) 7 mm/s, (c) 7.5 mm/s, (d) 8 mm/s.

(a)

(b)

6 mm/s

728.4 HV0.2 7.5 mm/s

846.6 HV0.2

7 mm/s

754.2 HV0.2 8 mm/s

804.3 HV0.2

Fig. 6. Microhardness profiles (a) and surface indentation morphology (b) of the coatings.

the resistance at the interface between the solution and the substrate in parallel with the CPEt with constant phase angle, which is used to depict the capacitive arc in electrochemical impedance spectroscopy (EIS). In Fig. 8b, Rs represents the resistance of NaCl solution, Rb is the passivation film resistance parallel to the constant phase angle CPEb, and Rt is the barrier layer resistance of the coating parallel to the constant phase angle CPEt. There are two capacitive arcs in the Nyquist diagram of the substrate, the radius of capacitive arc is small at high frequency and large at low frequency, which indicates that the matrix has pitting corrosion in the solution [39,40]. In Fig. 8b, the coating exhibits a higher total impedance. With the increase of scanning speed,

view of corrosion rate, the corrosion rate is proportional to the selfcorrosion current density, and the lower icorr, indicates the lower corrosion rate, which is consistent with the corrosion rate measured in practice [38]. When the scanning speed is 7.5 mm/s, the corrosion rate is the lowest, which is 0.0725 mm·a−1. Compared with the substrate, the corrosion resistance of the substrate is greatly improved. Fig. 11 shows the electrochemical impedance spectroscopy (Nyquist) diagram of the substrate and coating, in which Z′ is the real part of the impedance, Z″ is the imaginary part of the impedance, and the lower right corner of the figure is the corresponding fitting equivalent circuit. In Fig. 8a, Rs represents the resistance of NaCl solution, and Rt is

23

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Fig. 7. Residual stress of coating Surface with different scanning speed: (a) 6 mm/s, (b) 7 mm/s, (c) 7.5 mm/s, (d) 8 mm/s.

than that of the substrate barrier layer. At the same time, the capacitance Qt of the coating is obviously one order of magnitude lower than that of the substrate, so the coating shows good corrosion resistance to the substrate. The coating with scanning speed of 7.5 mm/s has the best corrosion resistance. In the process of rapid solidification by laser cladding, the solidification and cooling rate of the molten pool is fast, so the microstructure of the coating is fine, and the grain orientation of the directional solidification structure is consistent, which shortens the reaction process of the primary battery. On the other hand, the solid solubility of AlNi3, TiC and LaAl3 in the microstructure is increased, which makes the coating more stable, thus improving the corrosion resistance of the substrate. Fig. 12 shows a three-dimensional topography of the coating and substrate surface after electrochemical testing. The red color in the figure indicates the corrosion zone. The result shows that the corrosion area of the substrate surface is larger and there are obvious impact piercings, which are mainly due to the lower pitting corrosion potential and poor corrosion resistance of the substrate. For the composite coating, it can be seen that the coating surface shows localized corrosion characteristics. This is due to the existence of local cracks and pores on the surface of the coating and the preferential breakdown of the defects in the coating during electrochemical testing, which makes the Cl− enter into the coating, leading to the gradual destruction of the coating and the gradual reduction of corrosion resistance [44,45]. When the scanning speed reaches 7.5 mm/s, the corrosion resistance of the coating is enhanced because of less pores and cracks,

Fig. 8. Friction coefficients of the substrate and coatings versus time.

the maximum impedance of the coating can reach about 25,000 Ω. At this time, a very large capacitive reactance arc appears in the high frequency region, and the capacitance loop radius is larger. Therefore, the corrosion resistance is better [41–43]. Table 5 is a fitting result based on the EIS circuit diagram. The Rt of the coatings prepared at different scanning rates is 1 kΩ·cm2, 2.408 kΩ·cm2, 6.670 kΩ·cm2, 6.290 kΩ·cm2, respectively, which is one order of magnitude higher 24

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Fig. 9. Profile of worn track on coatings with different scanning speeds: (a) 6 mm/s, (b) 7 mm/s, (c) 7.5 mm/s, (d) 8 mm/s. Table 3 Wear test results of coatings. Scanning speeds (mm/s)

Wear width (μm)

Wear depth (μm)

Wear area (mm2)

Wear volume (mm3)

Wear rate (mm3·N−1·s−1)

6 7 7.5 8

558.3 547.7 449.4 537.3

37.9 36.4 15.5 26.9

11.50 ± 0.1 11.26 ± 0.1 9.10 ± 0.1 11.03 ± 0.1

0.43 0.41 0.14 0.29

1.19 × 10−5 1.13 × 10−5 3.88 × 10−6 8.05 × 10−6

± ± ± ±

1 1 1 1

± ± ± ±

0.1 0.1 0.1 0.1

± ± ± ±

0.01 0.01 0.01 0.01

Table 4 Electrochemical data of substrate and coatings at different scanning speeds. Sample 6.0 mm/s 7.0 mm/s 7.5 mm/s 8.0 mm/s Substrate

Icorr(A/cm2)

Ecorr(V) −0.76 −0.75 −0.60 −0.71 −0.80

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

Corrosion rate(mm/a) −7

(6.88 ± 0.001) × 10 (3.36 ± 0.001) × 10−7 (2.375 ± 0.001) × 10−8 (6.274 ± 0.001) × 10−8 (2.770 ± 0.001) × 10−6

0.138 0.126 0.072 0.078 0.215

± ± ± ± ±

0.001 0.001 0.001 0.001 0.001

higher density and higher resistance required for breakdown. 4. Conclusions (1) The Ni/TiC/La2O3 reinforced Al based composite coating was prepared on the surface of S355 offshore steel. The coating was mainly composed of reinforced phase TiC and matrix continuous phase. The dilution rate of the coating decreased with the increase of scanning speed, and the coating exhibited good metallurgical bonding with the substrate. As the scanning speed increases, the microstructure of the coating changed gradually from short rod-like to granular, and the strengthening effect of fine grain was obvious. (2) With the increase of the scanning speed, the microhardness of the coating increases at first and then decreases slightly. At the distance of 100 μm from the surface, the microhardness of the coating

Fig. 10. Potentiodynamic polarization of substrate and coatings at different scanning speeds.

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Fig. 11. Nyquist of substrate (a) and coatings (b) at different scanning speeds. Table 5 EIS data of substrate and coatings at different scanning speeds. Sample

Rs (Ω·cm2)

Qb (Ω−1·s−n·cm−2)

Nb

Rb (Ω·cm2)

Qt (Ω−1·s−n·cm−2)

Nt

Rt (Ω·cm2)

Substrate 6.0 mm/s 7.0 mm/s 7.5 mm/s 8.0 mm/s

16.26 3.427 6.08 10 10

– 1.734 × 10−6 5.452 × 10−4 2.278 × 10−4 6.756 × 10−4

– 0.7939 0.606 0.4502 0.3457

– 14.85 29.62 17.24 19.93

1.069 × 10−3 5.898 × 10−4 5.716 × 10−4 1.007 × 10−4 1.027 × 10−4

0.8 0.6201 1 0.72 0.8519

685 1000 2408 6670 6290

Fig. 12. Corrosion morphology of substrate (a) and coating surface (b) with different scanning speeds: (b1) 6 mm/s, (b2) 7 mm/s, (b3) 7.5 mm/s, (b4) 8 mm/s. 26

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reaches the maximum value of 894 HV0.2, which is about 2.5 times of that of the substrate. The residual stress on the coating surface is tensile stress. When the scanning speed is 7.5 mm/s, the wear resistance of the coating is the best, and the wear-reducing ability of the coating is about 2 times of that of the substrate. (3) The impedance and corrosion resistance of the coatings prepared at different scanning speeds are higher than that of the substrate. When the scanning speed is 7.5 mm/s, the coatings show the best corrosion resistance.

[20] [21]

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Acknowledgments [25]

The authors gratefully acknowledge the financial support from the Key Research and Development Project of Jiangsu Province (BE2016052) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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